Method for Preparing Super Absorbent Polymer

ABSTRACT

The present disclosure relates to a method for preparing a super absorbent polymer. More specifically, the above method performs a polymerization reaction of a monomer in the presence of an aqueous dispersion of hydrophobic particles, and thus a super absorbent polymer having an improved absorption rate can be prepared without deterioration in absorption properties.

CROSS-REFERENCE TO RELATED APPLICATIONS

This present application is a national stage entry under 35 U.S.C. § 371of International Application No. PCT/KR2021/019244 filed on Dec. 17,2021, which claims priority from Korean Patent Applications No.10-2020-0178433 filed on Dec. 18, 2020 and No. 10-2021-0180293 filed onDec. 16, 2021, all the disclosures of which are incorporated herein byreference.

TECHNICAL FIELD

The present disclosure relates to a method for preparing a superabsorbent polymer. More specifically, it relates to a method forpreparing a super absorbent polymer having a high surface tension and animproved absorption rate without deterioration in absorption properties,because the generated carbon dioxide bubbles can be effectively capturedby performing a polymerization reaction of a monomer in the presence ofan aqueous dispersion of hydrophobic particles.

BACKGROUND OF ART

A super absorbent polymer (SAP) is a type of synthetic polymericmaterial capable of absorbing 500 to 1000 times its own weight ofmoisture. Various manufacturers have denominated it with differentnames, such as SAM (Super Absorbency Material), AGM (Absorbent GelMaterial), and the like. Such super absorbent polymers started to bepractically applied in sanitary products, and they are now being widelyused not only for hygiene products, but also for water retaining soilproducts for gardening, water stop materials for the civil engineeringand construction, sheets for raising seedling, fresh-keeping agents forfood distribution fields, materials for poultices, or the like.

These super absorbent polymers have been widely used in the field ofhygienic materials such as diapers or sanitary napkins. In such hygienicmaterials, the super absorbent polymer is generally contained in a stateof being spread in the pulp. In recent years, however, continuousefforts have been made to provide hygienic materials such as diapershaving a thinner thickness. As a part of such efforts, the developmentof so-called pulpless diapers and the like in which the pulp content isreduced or pulp is not used at all is being actively advanced.

As described above, in the case of hygienic materials in which the pulpcontent is reduced or the pulp is not used, a super absorbent polymer iscontained at a relatively high ratio and these super absorbent polymerparticles are inevitably contained in multiple layers in the hygienicmaterials. In order for the whole super absorbent polymer particlescontained in the multiple layers to more efficiently absorb a largeamount of liquid such as urine, it is necessary for the super absorbentpolymer to exhibit high absorption performance as well as fastabsorption rate.

In order to prepare the super absorbent polymer with improved absorptionrate, a method of increasing a specific surface area by using a foamingagent in the polymerization step to form pores is mainly used. Inparticular, an encapsulated foaming agent is commonly used in terms ofprice and availability, and when the polymerization step is performed inthe presence of such an encapsulated foaming agent, carbon dioxidebubbles are generated and the specific surface area in the cross-linkedpolymer increases. In addition, a bubble stabilizer is used to minimizethe generated carbon dioxide bubbles from escaping out of thecross-linked polymer network. However, the use of the bubble stabilizercauses a problem in that general properties of the super absorbentpolymer are deteriorated.

Accordingly, there is a continuous demand for the development of a superabsorbent polymer having a fast absorption rate while maintainingcentrifuge retention capacity (CRC), which is the property indicatingbasic absorbency and water retention capacity of the super absorbentpolymer, and absorbency under pressure (AUP), which is the property ofwell retaining the absorbed liquid even under external pressure.

DETAILED DESCRIPTION OF THE INVENTION Technical Problem

Accordingly, the present disclosure relates to a method for preparing asuper absorbent polymer capable of preparing a super absorbent polymerhaving an improved absorption rate by effectively capturing bubblesgenerated by a foaming agent after adding an aqueous dispersion ofhydrophobic particles in the polymerization step of a monomer.

Technical Solution

In order to solve the above problems, there is provided a method forpreparing a super absorbent polymer including the steps of:

-   -   preparing a monomer composition containing an acrylic acid-based        monomer having at least partially neutralized acidic groups and        an internal cross-linking agent (step 1);    -   preparing a hydrogel polymer by cross-linking polymerization of        the monomer composition in the presence of an aqueous dispersion        of hydrophobic particles and an encapsulated foaming agent (step        2);    -   forming a powder-type base resin by drying and pulverizing the        hydrogel polymer (step 3); and    -   forming a surface cross-linked layer by further cross-linking        the surface of the base resin in the presence of a surface        cross-linking agent (step 4),

wherein the aqueous dispersion of hydrophobic particles is a colloidalsolution in which hydrophobic particles are dispersed by a surfactant,

the hydrophobic particles contain a metal salt of a C7 to C24 fattyacid, and

the encapsulated foaming agent has a structure having a core including ahydrocarbon and a shell formed of a thermoplastic resin surrounding thecore.

Advantageous Effects

According to the method for preparing a super absorbent polymer of thepresent disclosure, when the hydrogel polymer is prepared bycross-linking polymerization of a monomer in the presence of an aqueousdispersion of hydrophobic particles and an encapsulated foaming agent,the generated carbon dioxide can be effectively captured and thespecific surface area of the super absorbent polymer can be increased.Accordingly, the absorption rate of the super absorbent polymer to beprepared may be increased.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to limit the invention. Thesingular forms are intended to include the plural forms as well, unlessthe context clearly indicates otherwise. It will be further understoodthat the terms “include”, “have”, or “possess” when used in thisspecification, specify the presence of stated features, steps,components, or combinations thereof, but do not preclude the presence oraddition of one or more other features, steps, components, orcombinations thereof.

As the present invention can be variously modified and have variousforms, specific embodiments thereof are shown by way of examples andwill be described in detail. However, it is not intended to limit thepresent invention to the particular form disclosed and it should beunderstood that the present invention includes all modifications,equivalents, and replacements within the idea and technical scope of thepresent invention.

In addition, the terminologies are used merely to refer to specificembodiments, and are not intended to restrict the present disclosure.Singular expressions of the present disclosure may include pluralexpressions unless they are differently expressed contextually.

The terminology “polymer” in the present disclosure is in a state inwhich a water-soluble ethylene-based unsaturated monomer is polymerized,and may include all moisture content ranges, or all particle diameterranges. Among the polymers, a polymer having a moisture content of about40 wt % or more after polymerization and before drying may be referredto as a hydrogel polymer, and particles in which the hydrogel polymer ispulverized and dried may be referred to as a cross-linked polymer.

In addition, the terminology “super absorbent polymer particle” refersto a particulate material containing a cross-linked polymer in which anacrylic acid-based monomer having at least partially neutralized acidicgroups is polymerized and cross-linked by an internal cross-linkingagent.

In addition, the terminology “super absorbent polymer” is used toencompass all of a cross-linked polymer in which an acrylic acid-basedmonomer having at least partially neutralized acidic groups ispolymerized or a base resin in the form of powder consisting of superabsorbent polymer particles in which the cross-linked polymer ispulverized, and the cross-linked polymer or the base resin furtherprocessed, for example, surface cross-linking, fine reassembling,drying, pulverization, classification, etc., to be in a state suitablefor commercialization, depending on the context. Accordingly, theterminology “super absorbent polymer” may be interpreted as including aplurality of super absorbent polymer particles.

In order to prepare a super absorbent polymer having a fast absorptionrate, the specific surface area in the super absorbent polymer particlesneeds to be increased. As a method for increasing the specific surfacearea of the super absorbent polymer particles, a foaming agent isusually used. In order for the bubbles generated by the foaming agent tocontribute to the increase in the surface area, they must be capturedinside the polymer as soon as they are generated so as not to escape outof the cross-linked polymer. For this purpose, a foaming agent and afoam stabilizer are usually added together. An anionic surfactant, whichis mainly used as the foam stabilizer, had to be used together with anexcess of a foaming agent in order to effectively capture bubbles.However, when such an anionic surfactant is used in excess, the surfacetension of the super absorbent polymer is lowered, pores in the superabsorbent polymer are not formed uniformly and evenly, and a largeamount of fines are generated during manufacture and transport.

Accordingly, the present inventors have found that in the case of usingan aqueous dispersion containing hydrophobic particles as a foamstabilizer in addition to the conventional foam stabilizer such as ananionic surfactant to prepare a super absorbent polymer, it effectivelygenerates and captures bubbles by acting as a seed when the bubbles aregenerated by the foaming agent, and the small and uniformly shapedbubbles can be uniformly distributed over the entire area of thecross-linked polymer, thereby completing the present invention.

In particular, the method for preparing a super absorbent polymer ischaracterized in that the hydrophobic particles are not used in the formof a powder, but are added to the monomer composition in the form of anaqueous dispersion. In other words, the hydrophobic particles areintroduced into the monomer composition in the form of “aqueousdispersion of hydrophobic particles”, that is, in the form of acolloidal solution in which the hydrophobic particles are stablydispersed without being precipitated or agglomerated by the surfactant.This is because, when the polymerization process is performed byintroducing the hydrophobic particles into the monomer composition inthe form of a powder, the hydrophobic particles are not dispersed in themonomer composition in the form of an aqueous solution, so that thebubbles generated by the foaming agent cannot be effectively stabilized.

In addition, the hydrophobic particles are stably dispersed in theaqueous dispersion by the surfactant without agglomeration betweenparticles. Specifically, the surfactant may form an electric doublelayer on the surface of hydrophobic particles to induce an electrostaticrepulsive force between particles, which may stabilize the hydrophobicparticles, or the surfactant may be adsorbed on the surface ofhydrophobic particles to induce a steric repulsive force betweenparticles, which may prevent the particles from agglomerating with eachother. Therefore, when a surfactant is not included in the aqueousdispersion of hydrophobic particles, a phenomenon in which thehydrophobic particles agglomerate with each other or sink due to gravityis caused, so that dispersion of the hydrophobic particles cannot bestabilized. Accordingly, even when an aqueous dispersion of hydrophobicparticles that does not contain a surfactant is used together with afoaming agent in the polymerization step, it is impossible toeffectively capture bubbles and thus pores having a uniform size cannotbe formed in the super absorbent polymer, so it is difficult to improvethe absorption rate of the super absorbent polymer.

Moreover, when an epoxy-based compound is used as an internalcross-linking agent in the preparation of the super absorbent polymer,the gelation time is slower than the case of using an acrylate-basedcompound as an internal cross-linking agent. Accordingly, there has beena problem in that the bubbles generated by the foaming agent before thecross-linked structure is formed are blown away, and thus the gas cannotbe effectively captured. Therefore, in order to increase the specificsurface area of the super absorbent polymer prepared by using anepoxy-based compound as an internal cross-linking agent, an excessiveamount of the foaming agent has to be used. However, when the aqueousdispersion of hydrophobic particles is used together with a foamingagent, even if only a small amount of the foaming agent is used, thehydrophobic particles function as a seed to effectively generate andcapture bubbles. Thus, it is possible to form a desired pore structurewithout the use of an excessive amount of the foaming agent.

In addition, the super absorbent polymer prepared according to oneembodiment has a vortex time at 24.0° C. of 35 seconds or less, andone-minute tap water absorbency, which is defined as a weight of waterabsorbed in the super absorbent polymer in 1 minute when 1 g of thesuper absorbent polymer is immersed in 2 L of tap water and swollen for1 minute, of 115 g/g or more.

Hereinafter, each step of the method for preparing a super absorbentpolymer according to a specific embodiment of the present disclosurewill be described in more detail.

(Step 1) In the preparation method according to one embodiment, step 1is a step of preparing a monomer composition containing an acrylicacid-based monomer having at least partially neutralized acidic groupsand an internal cross-linking agent.

The acrylic acid-based monomer is a compound represented by thefollowing Chemical Formula 1:

R¹—COOM¹   [Chemical Formula 1]

in Chemical Formula 1,

R¹ is a C2 to C5 alkyl group having an unsaturated bond, and

M¹ is a hydrogen atom, a monovalent or divalent metal, an ammoniumgroup, or an organic amine salt.

Preferably, the acrylic acid-based monomer may include at least oneselected from the group consisting of acrylic acid, methacrylic acid,and a monovalent metal salt, a divalent metal salt, an ammonium salt andan organic amine salt thereof.

Herein, the acrylic acid-based monomers may be those having acidicgroups which are at least partially neutralized. Preferably, the acrylicacid-based monomer partially neutralized with an alkali substance suchas sodium hydroxide, potassium hydroxide, ammonium hydroxide, or thelike may be used. A degree of neutralization of the acrylic acid-basedmonomer may be 40 to 95 mol %, 40 to 80 mol %, or 45 to 75 mol %. Therange of the degree of neutralization can be adjusted according to finalproperties. An excessively high degree of neutralization causes theneutralized monomers to be precipitated, and thus polymerization may notreadily occur. On the contrary, an excessively low degree ofneutralization not only deteriorates absorbency of the polymer, but alsogives the polymer hard-to-handle properties, such as those of an elasticrubber.

In addition, a concentration of the acrylic acid-based monomer may beabout 20 to 60 wt %, or about 40 to 50 wt % based on the monomercomposition containing the raw materials of the super absorbent polymerand the solvent, and properly controlled in consideration ofpolymerization time and reaction conditions. When the concentration ofthe monomer is excessively low, the yield of the super absorbent polymeris low and there may be a problem in economic efficiency. In contrast,when the concentration is excessively high, it may cause problems inprocesses in that some of the monomer may be extracted or thepulverization efficiency of the polymerized hydrogel polymer may belowered in the pulverization process, and thus physical properties ofthe super absorbent polymer may be deteriorated.

In addition, the terminology ‘internal cross-linking agent’ used hereinis different from a surface cross-linking agent for cross-linking thesurface of the super absorbent polymer particles to be described later,and the internal cross-linking agent polymerizes unsaturated bonds ofthe water-soluble ethylene-based unsaturated monomers by cross-linking.The cross-linking in the above step proceeds both on the surface and onthe inside, but when the surface cross-linking process of the superabsorbent polymer particles to be described later is in progress, thesurface of the particles of the finally prepared super absorbent polymerhas a structure cross-linked by a surface cross-linking agent, and theinside of the particles has a structure cross-linked by the internalcross-linking agent.

As the internal cross-linking agent, any compound may be used as long asit allows the introduction of cross-linking bonds during polymerizationof the acrylic acid-based monomer. Specifically, the internalcross-linking agent may be a cross-linking agent having one or moreethylene-based unsaturated groups in addition to the functional groupwhich may react with the water-soluble substituents of the acrylicacid-based monomer; or a cross-linking agent having two or morefunctional groups which may react with the water-soluble substituents ofthe monomer and/or the water-soluble substituents formed by hydrolysisof the monomer.

For example, as the internal cross-linking agent, a multifunctionalcross-linking agent may be used alone or in combination of two or more.Specifically, examples of the internal cross-linking agent include anacrylate-based compound such as N,N′-methylenebisacrylamide,trimethylpropane tri(meth)acrylate, ethylene glycol di(meth)acrylate,polyethylene glycol (meth)acrylate, polyethylene glycoldi(meth)acrylate, propylene glycol di(meth)acrylate, polypropyleneglycol (meth)acrylate, butanediol di(meth)acrylate, butylene glycoldi(meth)acrylate, diethylene glycol di(meth)acrylate, hexanedioldi(meth)acrylate, triethylene glycol di(meth)acrylate, tripropyleneglycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate,dipentaerythritol pentaacrylate, glycerin tri(meth)acrylate, andpentaerythritol tetraacrylate; an epoxy-based compound such as ethyleneglycol diglycidyl ether, diethylene glycol diglycidyl ether,polyethylene glycol diglycidyl ether, propylene glycol diglycidyl ether,tripropylene glycol diglycidyl ether, polypropylene glycol diglycidylether, neopentyl glycol diglycidyl ether, 1,6-hexanediol diglycidylether, polytetramethylene glycol diglycidyl ether, glycerol diglycidylether, glycerol triglycidyl ether, diglycerol polyglycidyl ether andpolyglycerol polyglycidyl ether; triarylamine; propylene glycol;glycerin; and ethylene carbonate, but the present disclosure is notlimited thereto.

According to one embodiment, the epoxy-based compound may be used as theinternal cross-linking agent. For example, as the internal cross-linkingagent, a polyvalent epoxy compound of divalent or higher such asethylene glycol diglycidyl ether may be used. In this case, foaming bythe foaming agent may be stably performed by the hydrophobic particles.

In the monomer composition, the internal cross-linking agent may be usedin an amount of 0.01 to 5 parts by weight based on 100 parts by weightof the acrylic acid-based monomer. For example, the internalcross-linking agent may be used in an amount of 0.01 parts by weight ormore, 0.05 parts by weight or more, 0.1 parts by weight or more, or 0.15parts by weight or more, and 5 parts by weight or less, 3 parts byweight or less, 2 parts by weight or less, 1 parts by weight or less, or0.7 parts by weight or less based on 100 parts by weight of thewater-soluble ethylene-based unsaturated monomer. When too littleinternal cross-linking agent is used, cross-linking does not occursufficiently, and thus it may be difficult to achieve strength above anappropriate level, and when too much internal cross-linking agent isused, the internal cross-linking density increases, and thus it may bedifficult to achieve a desired level of water retention capacity.

In addition, the monomer composition may further include apolymerization initiator for initiating a polymerization reaction of themonomer. The polymerization initiator is not particularly limited aslong as it is generally used in the preparation of super absorbentpolymers.

Specifically, the polymerization initiator may be an initiator forthermal polymerization or an initiator for photopolymerization by UVradiation according to the polymerization method. However, even when thephotopolymerization method is applied thereto, a certain amount heat isgenerated by UV radiation and the like, and some heat occurs as thepolymerization reaction, an exothermal reaction, progresses. Therefore,the composition may additionally include the thermal polymerizationinitiator.

More specifically, any compound which can form a radical by light suchas UV rays may be used as the photopolymerization initiator withoutlimitation.

For example, the photopolymerization initiator may be one or morecompounds selected from the group consisting of benzoin ether, dialkylacetophenone, hydroxyl alkylketone, phenyl glyoxylate, benzyl dimethylketal, acyl phosphine, and α-aminoketone. Further, as the specificexample of acyl phosphine, commercial lucirin TPO, namely,diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, may be used. Morevarious photopolymerization initiators are well disclosed in “UVCoatings: Basics, Recent Developments and New Application (Elsevier,2007)” written by Reinhold Schwalm, p 115, and the present disclosure isnot limited thereto.

The photopolymerization initiator may be used at a concentration ofabout 0.01 to about 1.0 wt % based on the monomer composition. When theconcentration of the photopolymerization initiator is excessively low,the polymerization rate may become slow, and when the concentration isexcessively high, the molecular weight of the super absorbent polymermay become low and properties may be uneven.

Furthermore, as the thermal polymerization initiator, one or moreinitiators selected from the group consisting of a persulfate-basedinitiator, an azo-based initiator, hydrogen peroxide, and ascorbic acidmay be used. Specifically, sodium persulfate (Na₂S₂O₈), potassiumpersulfate (K₂S₂O₈), ammonium persulfate ((NH₄)₂S₂O₈), and the like maybe used as examples of the persulfate-based initiators; and2,2-azobis-(2-amidinopropane)dihydrochloride,2,2-azobis-(N,N-dimethylene)isobutyramidinedihydrochloride,2-(carbamoylazo)isobutylonitril,2,2-azobis-[2-(2-imidazolin-2-yl)propane]dihydrochloride,4,4-azobis-(4-cyanovaleric acid), and the like may be used as examplesof azo-based initiators. More various thermal polymerization initiatorsare well disclosed in “Principle of Polymerization (Wiley, 1981)”written by Odian, p 203, and the present disclosure is not limitedthereto.

The thermal polymerization initiator may be used at a concentration ofabout 0.001 to about 0.5 wt % based on the monomer composition. When theconcentration of the thermal polymerization initiator is excessivelylow, additional thermal polymerization hardly occurs and there may beless effect of adding the thermal polymerization initiator. When theconcentration of the thermal polymerization initiator is excessivelyhigh, the molecular weight of the super absorbent polymer may become lowand the properties may be uneven.

The polymerization initiator may be used in an amount of 2 parts byweight or less based on 100 parts by weight of the water-solubleethylene-based unsaturated monomer. When the concentration of thepolymerization initiator is excessively low, the polymerization rate maybecome slow, and a large amount of residual monomers may be extractedfrom the final product. Conversely, when the concentration of thepolymerization initiator is higher than the above range, polymer chainsforming a network are shortened, so that the content of extractablecomponents increases and absorbency under pressure decreases, therebylowering physical properties of the polymer.

The monomer composition may further include an additive such as athickener, a plasticizer, a preservation stabilizer, and an antioxidant,if necessary.

In addition, the monomer composition containing the monomer may be, forexample, in the form of a solution dissolved in a solvent such as water.The solid content of the monomer composition in a solution state, thatis, the concentration of the monomer, the internal cross-linking agent,and the polymerization initiator may be appropriately adjusted inconsideration of the polymerization time and reaction conditions. Forexample, the solid content of the monomer composition may be 10 to 80 wt%, 15 to 60 wt %, or 30 to 50 wt %.

When the monomer composition has the solid content in the above range,it may be advantageous for controlling the pulverization efficiencyduring pulverization of the polymer to be described later whileeliminating the need to remove unreacted monomers after polymerizationby using a gel effect phenomenon occurring in the polymerizationreaction of a high-concentration aqueous solution.

At this time, any solvent which can dissolve the above components may beused without limitation. For example, the solvent may be at least oneselected from water, ethanol, ethyleneglycol, diethyleneglycol,triethyleneglycol, 1,4-butanediol, propyleneglycol, ethyleneglycolmonobutylether, propyleneglycol monomethylether, propyleneglycolmonomethylether acetate, methylethylketone, acetone, methylamylketone,cyclohexanone, cyclopentanone, diethyleneglycol monomethylether,diethyleneglycol ethylether, toluene, xylene, butyrolactone, carbitol,methylcellosolve acetate, or N,N-dimethylacetamide.

(Step 2)

Subsequently, a step of preparing a hydrogel polymer by cross-linkingpolymerization of the monomer composition in the presence of an aqueousdispersion of hydrophobic particles and an encapsulated foaming agent isperformed. As the capsule of the encapsulated foaming agent bursts inthe above step, bubbles are generated by the hydrocarbon gas in thecapsule, and the hydrophobic particles dispersed in water effectivelycapture these bubbles, thereby increasing the specific surface area ofthe prepared hydrogel polymer.

The hydrophobic particles contain a metal salt of a C7 to C24 fattyacid. Herein, the metal salt of a C7 to C24 fatty acid refers to acompound in which a metal cation is bonded instead of a hydrogen ion ofa carboxyl group at the end of an unsaturated or saturated fatty acidhaving a linear structure while having 7 to 24 carbon atoms in themolecule, and may be a monovalent metal salt, or a polyvalent metal saltof divalent or higher. At this time, when the hydrophobic particles area metal salt of a fatty acid having less than 7 carbon atoms, it is notpossible to capture the bubbles generated in the form of particles byionization in an aqueous solution. When the hydrophobic particles are ametal salt of a fatty acid having more than 24 carbon atoms, the chainof the fatty acid becomes long, which may cause difficult dispersion.

Specifically, when the metal salt of the fatty acid is a monovalentmetal salt, it has a structure in which one fatty acid carboxylate anionis bonded to an alkali ion, which is a monovalent metal cation. Inaddition, when the metal salt of the fatty acid is a polyvalent metalsalt of divalent or higher, it has a structure in which as many as fattyacid carboxylate anions as the number of the valence of the metal cationare bonded to the metal cation.

In one embodiment, the hydrophobic particles may be a metal salt of aC12 to C20 saturated fatty acid. For example, the hydrophobic particlesmay be at least one metal salt of a saturated fatty acid selected fromthe group consisting of a metal salt of lauric acid containing 12 carbonatoms in the molecule; a metal salt of tridecyl acid containing 13carbon atoms in the molecule; a metal salt of myristic acid containing14 carbon atoms in the molecule; a metal salt of pentadecanoic acidcontaining 15 carbon atoms in the molecule; a metal salt of palmiticacid containing 16 carbon atoms in the molecule; a metal salt ofmargaric acid containing 17 carbon atoms in the molecule; a metal saltof stearic acid containing 18 carbon atoms in the molecule; a metal saltof nonadecylic acid containing 19 carbon atoms in the molecule; and ametal salt of arachidic acid containing 20 carbon atoms in the molecule.

Preferably, the hydrophobic particles may be a metal salt of stearicacid. For example, the hydrophobic particles may be at least one metalsalt of stearic acid selected from the group consisting of calciumstearate, magnesium stearate, sodium stearate, zinc stearate andpotassium stearate.

In addition, as described above, the hydrophobic particles dispersed inthe aqueous dispersion may have an average particle diameter of 0.5 μmto 20 μm. When the hydrophobic particles have an average particlediameter of less than 0.5 μm, there is a problem in that it is difficultto effectively capture the generated bubbles, so that uniform porescannot be formed. When the hydrophobic particles have an averageparticle diameter of more than 20 μm, the pore size to be formed becomesexcessively large, so it may be difficult to improve the absorption rateof the super absorbent polymer. Specifically, the hydrophobic particlesmay have an average particle diameter (μm) of 0.5 or more, 1 or more, 2or more, or 3 or more, and 20 or less, 15 or less, or 10 or less.

Herein, the average particle diameter of the hydrophobic particles meansD50, and the “particle diameter Dn” means a particle diameter at the n %point of the cumulative distribution of the number of particlesaccording to particle diameters. In other words, D50 is a particlediameter at the 50% point of the cumulative distribution of the numberof particles according to particle diameters, D90 is a particle diameterat the 90% point of the cumulative distribution of the number ofparticles according to particle diameters, and D10 is a particlediameter at the 10% point of the cumulative distribution of the numberof particles according to particle diameters. The Dn may be measuredusing a laser diffraction method. Specifically, the powder to bemeasured is dispersed in the dispersion medium and introduced into acommercially available particle size measuring device (e.g., MicrotracS3500). Then, a particle size distribution is obtained by measuring adifference in diffraction patterns according to particle diameters whenthe particles pass through the laser beam. In the measuring device, D10,D50 and D90 can be obtained by calculating a particle diameter at apoint of reaching 10%, 50% and 90% of the cumulative distribution of thenumber of particles according to particle diameters.

In addition, the hydrophobic particles may be included in the aqueousdispersion in an amount of 10 to 70 wt % based on a total weight of theaqueous dispersion. When the content of the hydrophobic particles in thehydrophobic aqueous dispersion is too low or too high, dispersionstabilization of the hydrophobic particles cannot be achieved, and aproblem of agglomeration between particles or sinkage by gravity mayoccur.

In addition, as the surfactant for dispersing the hydrophobic particlesin the aqueous dispersion of hydrophobic particles, a surfactant knownin the art that can stabilize the dispersion of the hydrophobicparticles may be used without limitation. For example, one or moresurfactants selected from the group consisting of cationic surfactants,anionic surfactants, amphoteric surfactants and nonionic surfactants maybe used as the surfactant. Preferably, two or more surfactants may beused for dispersion stabilization of the hydrophobic particles. Morespecifically, in consideration of the form of the hydrophobic particles,for example, the form of a metal salt of a saturated fatty acid, anonionic surfactant and an anionic surfactant may be used together inorder to more effectively disperse the hydrophobic particles in water.For example, a nonionic surfactant to which a long-chain hydrocarbonhaving 10 or more carbon atoms is bonded and a sulfate-based anionicsurfactant may be used together.

For example, examples of the cationic surfactant includedialkyldimethylammonium salt and alkylbenzylmethylammonium salt,examples of the anionic surfactant include alkylpolyoxyethylene sulfate,monoalkyl sulfate, alkylbenzene sulfonate, monoalkyl phosphate, asulfate having a functional group containing a long-chain hydrocarbon ora salt thereof such as sodium lauryl ether sulfate, ammonium laurylsulfate, sodium dodecyl sulfate, sodium myreth sulfate, or sodiumlaureth sulfate, examples of the amphoteric surfactant includealkylsulfobetaine and alkylcarboxybetaine, and examples of the nonionicsurfactant include polyoxyethylene alkyl ether such as polyethyleneglycol or polyoxyethylene lauryl ether, polyoxyalkylene alkylphenylether, polyoxyethylene arylphenyl ether, fatty acid ester such aspolysorbate, fatty acid sorbitan ester, or glycerin monostearate, alkylmonoglyceryl ether, alkanolamide, and alkyl polyglycoside. However, thepresent disclosure is not limited thereto.

In addition, the aqueous dispersion of hydrophobic particles may have apH of 7 or more. When the pH of the aqueous dispersion of hydrophobicparticles is less than 7, it is acidic, so it is difficult to stabilizethe hydrophobic particles, which are metal salts of fatty acids, whichis not suitable.

Meanwhile, the hydrophobic particles may be used in an amount of 0.005to 0.4 parts by weight based on 100 parts by weight of the acrylicacid-based monomer. When the content of the hydrophobic particles is toolow, the bubbles may not be sufficiently captured, and thus theabsorption rate may be slowed. When the content of the hydrophobicparticles is too high, the amount of the surfactant used to stabilizethe hydrophobic particles in the aqueous dispersion of the hydrophobicparticles increases, so that the surface tension may be lowered. Forexample, the hydrophobic particles may be used in an amount of 0.01parts by weight or more, 0.03 parts by weight or more, 0.05 parts byweight or more, or 0.1 parts by weight or more, and 0.35 parts by weightor less, 0.3 parts by weight or less, 0.25 parts by weight or less, or0.2 parts by weight or less based on 100 parts by weight of the acrylicacid-based monomer.

In addition, the encapsulated foaming agent refers to a thermallyexpandable microcapsule foaming agent having a core-shell structure, andthe core-shell structure has a core including a hydrocarbon and a shellformed of a thermoplastic resin on the core. Specifically, thehydrocarbon constituting the core is a liquid hydrocarbon having a lowboiling point and is easily vaporized by heat. Therefore, when heat isapplied to the encapsulated foaming agent, the thermoplastic resinconstituting the shell is softened and the liquid hydrocarbon of thecore is vaporized at the same time. In addition, as the pressure insidethe capsule increases, the encapsulated foaming agent expands, andaccordingly, bubbles having an increased size than the existing size areformed.

Accordingly, the encapsulated foaming agent generates hydrocarbon gas,and is distinguished from an organic foaming agent that generatesnitrogen gas through an exothermic decomposition reaction betweenmonomers participating in the production of a polymer, and an inorganicfoaming agent that foams carbon dioxide gas by absorbing heat generatedin the production of a polymer.

The encapsulated foaming agent may have different expansioncharacteristics depending on the components constituting the core andthe shell, and the weight and diameter of each component. Therefore, theencapsulated foaming agent can be expanded to a desired size byadjusting them, thereby controlling the porosity of the super absorbentpolymer.

Specifically, the encapsulated foaming agent has a particle shape havingan average diameter (D₀) of 5 to 30 μm before expansion. It is difficultto manufacture the encapsulated foaming agent to have an averagediameter of less than 5 μm. When the average diameter of theencapsulated foaming agent exceeds 30 μm, it may be difficult toefficiently increase the surface area because the size of pores is toolarge. Therefore, when the encapsulated foaming agent has the averagediameter as described above, it can be determined that the encapsulatedfoaming agent is suitable for achieving an appropriate pore structure inthe resin.

For example, the average diameter before expansion of the encapsulatedfoaming agent may be 5 μm or more, 6 μm or more, 7 μm or more, 8 μm ormore, or 10 μm or more, and 30 μm or less, 25 μm or less, 20 μm or less,17 μm or less, 16 μm or less, or 15 μm or less.

The average diameter (D₀) of the encapsulated foaming agent beforeexpansion can be measured by measuring the diameter of each encapsulatedfoaming agent particle as an average Feret diameter with an opticalmicroscope, and then obtaining an average value thereof.

In this case, a capsule thickness of the encapsulated foaming agent maybe 2 to 15 μm.

In addition, the encapsulated foaming agent has a maximum expansion sizein air of 20 to 190 μm. Herein, the “maximum expansion size of theencapsulated foaming agent” means a diameter range of the top 10 wt % ofthe highly expanded particles after applying heat to the encapsulatedfoaming agent. It is difficult to manufacture the encapsulated foamingagent such that the maximum expansion size in air is smaller than 20 μm,and when the maximum expansion size in air exceeds 190 μm, it may bedifficult to efficiently increase the surface area because the size ofpores is too large.

For example, the encapsulated foaming agent may have a maximum expansionsize in air of 50 to 190 μm, 70 to 190 μm, 75 to 190 μm, or 80 to 150μm.

The maximum expansion size in air of the encapsulated foaming agent maybe determined by applying 0.2 g of the encapsulated foaming agent on aglass Petri dish and leaving it on a hot plate preheated to 150° C. for10 minutes, and then observing the expanded encapsulated foaming agentwith an optical microscope. Then, it may be obtained by measuring thediameter of the top 10 wt % of the highly expanded particles as anaverage Feret diameter with an optical microscope.

In addition, the encapsulated foaming agent has a maximum expansionratio in air of 5 to 15 times. Herein, the “maximum expansion ratio ofthe encapsulated foaming agent” means a ratio (D_(M)/D₀) of the averagediameter (D_(M)) of the top 10 wt % of the highly expanded particlesafter applying heat to the average diameter (D₀) of the encapsulatedfoaming agent measured before applying heat. When the maximum expansionratio in air of the encapsulated foaming agent is less than 5 times, anappropriate pore structure cannot be formed in the super absorbentpolymer, so there is a problem in that it is impossible to manufacture asuper absorbent polymer with improved absorbency and absorption rate. Itis difficult to manufacture the encapsulated foaming agent such that themaximum expansion ratio in air exceeds 15 times, considering the averagediameter of the encapsulated foaming agent before expansion. Therefore,it can be determined that the encapsulated foaming agent having themaximum expansion ratio within the above range is suitable for forming apore structure suitable for the super absorbent polymer.

For example, the maximum expansion ratio in air of the encapsulatedfoaming agent may be 5 times or more, 7 times or more, or 8 times ormore, and 15 times or less, 13 times or less, 11 times or less, or 10times or less.

At this time, the average diameter (D₀) of the encapsulated foamingagent measured before applying heat may be measured as described above.In addition, the average diameter (D_(M)) of the top 10 wt % of thehighly expanded particles after applying heat may be determined byapplying 0.2 g of the encapsulated foaming agent on a glass Petri dishand leaving it on a hot plate preheated to 150° C. for 10 minutes, andthen observing the expanded encapsulated foaming agent with an opticalmicroscope. Then, it may be obtained by measuring the diameter of eachof the top 10 wt % of the particles as an average Feret diameter with anoptical microscope, and then obtaining an average value thereof.

The expansion characteristics of the encapsulated foaming agent may befurther specified in Examples to be described later.

The reason for measuring the maximum expansion size and the maximumexpansion ratio of the encapsulated foaming agent in air is to determinewhether pores having a desired size are formed in the super absorbentpolymer to be prepared using the encapsulated foaming agent.Specifically, the shape in which the foaming agent is foamed may varydepending on the preparation conditions of the super absorbent polymer,so it is difficult to define the foamed shape. Therefore, the expansionsize and the expansion ratio are determined by first foaming theencapsulated foaming agent in air, and confirming whether theencapsulated foaming agent is suitable for forming the desired pores.

And, the hydrocarbon constituting the core of the encapsulated foamingagent may be at least one selected from the group consisting ofn-propane, n-butane, iso-butane, cyclobutane, n-pentane, iso-pentane,cyclopentane, n-hexane, iso-hexane, cyclohexane, n-heptane, iso-heptane,cycloheptane, n-octane, iso-octane and cyclooctane. Among them, the C3to C5 hydrocarbons (n-propane, n-butane, iso-butane, cyclobutane,n-pentane, iso-pentane, cyclopentane) are suitable for forming poreshaving the above-mentioned size, and iso-butane may be most suitable.

In addition, the thermoplastic resin constituting the shell of theencapsulated foaming agent may be a polymer formed from at least onemonomer selected from the group consisting of (meth)acrylate-basedcompounds, (meth)acrylonitrile-based compounds, aromatic vinylcompounds, vinyl acetate compounds, and halogenated vinyl compounds.Among them, a copolymer of (meth)acrylate and (meth)acrylonitrile may bemost suitable for forming pores having the above-mentioned size.

In addition, the foaming start temperature (T_(start)) of theencapsulated foaming agent may be 60° C. to 120° C., 65° C. to 120° C.,or 70° C. to 80° C., and the maximum foaming temperature (T_(max)) maybe 100° C. to 160° C., 105° C. to 155° C., or 110° C. to 120° C. Withinthe above range, foaming may occur easily in a subsequent thermalpolymerization process or drying process to introduce a pore structurein the polymer. The foaming start temperature and the foaming maximumtemperature can be measured using a thermomechanical analyzer.

In addition, the encapsulated foaming agent may be used in an amount of0.005 to 1 part by weight based on 100 parts by weight of the acrylicacid-based monomer. When the content of the foaming agent is less than0.005 parts by weight, the effect of adding the foaming agent may beinsignificant. When the content of the foaming agent exceeds 1 part byweight, there are too many pores in the cross-linked polymer, so thatgel strength of the super absorbent polymer to be prepared decreases andthe density also decreases, which may cause problems in distribution andstorage. For example, the encapsulated foaming agent may be used in anamount of 0.01 parts by weight or more, 0.03 parts by weight or more, or0.05 parts by weight or more, and 0.8 parts by weight or less, 0.6 partsby weight or less, or 0.5 parts by weight or less based on 100 parts byweight of the acrylic acid-based monomer.

In addition, the encapsulated foaming agent and the hydrophobicparticles may be used in a weight ratio of 1:0.1 to 1:3.5. When thehydrophobic particles are used in an excessively low content compared tothe encapsulated foaming agent, it is difficult to effectively capturethe generated bubbles. When the hydrophobic particles are used in anexcessively high content compared to the foaming agent, various physicalproperties such as water retention capacity and absorption rate maydecrease. For example, the hydrophobic particles may be used in a weightratio of 0.1 times or more, 0.2 times or more, or 0.3 times or more, and3.5 times or less, 3.0 times or less, 2.5 times or less, 2.0 times orless, or 1.5 times or less relative to the weight of the encapsulatedfoaming agent.

In addition, a surfactant commonly used as a foam stabilizer may befurther added together with the encapsulated foaming agent and theaqueous dispersion of hydrophobic particles in the step 2. Herein, theusable surfactant may be cationic surfactants such as quaternaryammonium compounds, e.g., dodecyltrimethylammonium chloride, ordodecyltrimethylammonium bromide; anionic surfactants such as alkylsulfate-based compounds, e.g., sodium dodecyl sulfate, ammonium laurylsulfate, sodium lauryl ether sulfate, or sodium myreth sulfate; ornonionic surfactants such as alkyl ether sulfate-based compounds, e.g.,polyoxyethylene lauryl ether, but is not limited thereto.

More specifically, at least one foam stabilizer selected from the groupconsisting of an alkyl sulfate-based compound and a polyoxyethylenealkyl ether-based compound may be further added in the step 2 togetherwith an encapsulated foaming agent and the aqueous dispersion ofhydrophobic particles. When the foam stabilizer is further added,dispersibility of the encapsulated foaming agent and the aqueousdispersion of hydrophobic particles is improved, so that the generationand capture of bubbles can occur uniformly, and the absorption rate ofthe prepared super absorbent polymer can be increased.

In this case, the encapsulated foaming agent and the foam stabilizer maybe used in a weight ratio of 1:0.01 to 1:0.5. However, when the foamstabilizer is used too much, it may coat the base resin or the surfaceof the super absorbent polymer, thereby reducing processability andreducing absorption-related physical properties of the super absorbentpolymer, which is not preferable. For example, the foam stabilizer maybe used in a weight ratio of 0.015 times or more, 0.02 times or more, or0.05 times or more, and 0.4 times or less, 0.35 times or less, or 0.3times or less relative to the weight of the encapsulated foaming agent.

Meanwhile, the polymerization of the monomer composition in the presenceof such an aqueous dispersion of hydrophobic particles and anencapsulated foaming agent is not particularly limited as long as it isa commonly used polymerization method.

Specifically, the polymerization method is largely divided into thethermal polymerization and the photopolymerization according to anenergy source of the polymerization. In the case of thermalpolymerization, it is generally carried out in a reactor equipped withan agitation spindle, such as a kneader. In the case ofphotopolymerization, it may be carried out in a reactor equipped with amovable conveyor belt. However, the polymerization method is just anexample, and the present disclosure is not limited thereto.

For example, in the reactor equipped with an agitation spindle such as akneader, the hydrogel polymer obtained by thermal polymerization bysupplying hot air or heating the reactor may be discharged to a reactoroutlet in the form of several centimeters to several millimetersdepending on a shape of the agitation spindle provided in the reactor.Specifically, a size of the hydrogel polymer obtained may vary dependingon the concentration and injection rate of the monomer composition to beinjected, and a hydrogel polymer having a weight average particlediameter of 2 to 50 mm may be usually obtained.

In addition, when photopolymerization is performed in the reactorequipped with a movable conveyor belt as described above, a hydrogelpolymer in the form of a sheet having a belt width may usually beobtained. At this time, a thickness of the polymer sheet may varydepending on the concentration and injection rate of the monomercomposition to be injected, and it is preferable to supply the monomercomposition so that the polymer in the form of a sheet has a thicknessof about 0.5 to about 5 cm. When the monomer composition is supplied tosuch an extent that the thickness of the polymer sheet is too thin, theproduction efficiency may be low. When the thickness of the polymersheet exceeds 5 cm, the polymerization reaction may not occur evenlyover the entire thickness due to the excessively thick thickness.

Generally, the moisture content of the hydrogel polymer obtained by theabove method may be about 40 to about 80 wt %. At this time, “moisturecontent” in the present disclosure is the content of moisture in theentire weight of the polymer, and it means a value of which the weightof the dried polymer is subtracted from the weight of the polymer.Specifically, the moisture content is defined as a value calculated bymeasuring the weight loss due to moisture evaporation from the polymerin the process of increasing the temperature of the polymer for dryingthrough infrared heating. At this time, the drying condition formeasuring the moisture content is as follows: the temperature isincreased to about 180° C. and maintained at 180° C., and the totaldrying time is 20 minutes including 5 minutes of a heating step.

(Step 3)

Subsequently, a step of drying and pulverizing the hydrogel polymer toform a powder-type base resin is performed. If necessary, a coarsepulverization step may be further performed before drying to increasethe efficiency of the drying step.

Herein, the pulverizing machine used is not particularly limited.Specifically, it may include at least one selected from the groupconsisting of a vertical pulverizer, a turbo cutter, a turbo grinder, arotary cutter mill, a cutter mill, a disc mill, a shred crusher, acrusher, a chopper, and a disc cutter, but it is not limited thereto.

In the pulverization step, the polymer may be pulverized to have adiameter of about 2 to about 10 mm.

It is technically difficult to pulverize the hydrogel polymer to have adiameter of less than 2 mm because of its high moisture content, andthere may be a phenomenon that the pulverized particles cohere with eachother. Meanwhile, when the polymer is pulverized to have a diameter oflarger than 10 mm, the efficiency enhancing effect in the subsequentdrying step may be insignificant.

The drying is performed on the pulverized polymer as described above oron the polymer immediately after polymerization without thepulverization step. The drying temperature in the drying step may beabout 150 to about 250° C. When the drying temperature is less than 150°C., the drying time may become excessively long and physical propertiesof the super absorbent polymer to be finally formed may decrease. Whenthe drying temperature is more than 250° C., only the surface of thepolymer is excessively dried, fine powder may be generated in thesubsequent pulverization process, and physical properties of the finalsuper absorbent polymer may decrease. Therefore, the drying maypreferably be performed at a temperature of about 150 to about 200° C.,more preferably at a temperature of about 160 to about 180° C.

Meanwhile, the drying time may be about 20 minutes to about 90 minutesin consideration of process efficiency, but is not limited thereto.

The drying method in the drying step is not particularly limited if ithas been generally used in the drying process of the hydrogel polymer.Specifically, the drying step may be performed by the method of hot airprovision, infrared radiation, microwave radiation, UV ray radiation,and the like. After the drying step, the moisture content of the polymermay be about 5 to about 10 wt %.

Subsequently, a step of pulverizing the dried polymer obtained throughthe drying step is performed.

The base resin, which is a polymer powder obtained after thepulverization step, may have a particle diameter of about 150 to about850 μm. As the pulverizing machine used for pulverization to such aparticle diameter, a pin mill, a hammer mill, a screw mill, a roll mill,a disc mill, a jog mill, or the like may be used, but the presentdisclosure is not limited thereto.

In order to manage the physical properties of the super absorbentpolymer powder to be commercialized after the pulverization step, thebase resin obtained after pulverization is classified according toparticle size. Preferably, the polymer having a particle diameter ofabout 150 to about 850 μm is classified, and only the base resin havingsuch a particle diameter may be subjected to a surface cross-linkingreaction step. In this case, the particle diameter may be measured inaccordance with the EDANA (European Disposables and NonwovensAssociation) WSP 220.3.

(Step 4)

Subsequently, a step of forming a surface cross-linked layer by furthercross-linking the surface of the base resin in the presence of a surfacecross-linking agent is performed. By the above step, there is provided asuper absorbent polymer in which a surface cross-linked layer is formedon the surface of the base resin, more specifically, on at least a partof the surface of each of the super absorbent polymer particles.

The surface cross-linking is a step of increasing a cross-linkingdensity near the surface of super absorbent polymer particles withregard to a cross-linking density inside the particles. Generally,surface cross-linking agents are applied on the surface of superabsorbent polymer particles. Therefore, surface cross-linking reactionsoccur on the surface of the super absorbent polymer particles, whichimproves cross-linkability on the surface of the particles withoutsubstantially affecting the inside of the particles. Thus, the surfacecross-linked super absorbent polymer particles have a higher degree ofcross-linking at the surface than inside.

As the surface cross-linking agent, any surface cross-linking agent thathas been conventionally used in the preparation of a super absorbentpolymer may be used without any particular limitation. Examples of thesurface cross-linking agent may include at least one polyol selectedfrom the group consisting of ethylene glycol, propylene glycol,1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, 1,2-hexanediol,1,3-hexanediol, 2-methyl-1,3-propanediol, 2,5-hexanediol,2-methyl-1,3-pentanediol, 2-methyl-2,4-pentanediol, tripropylene glycoland glycerol; at least one carbonate-based compound selected from thegroup consisting of ethylene carbonate, propylene carbonate, andglycerol carbonate; an epoxy compound such as ethylene glycol diglycidylether; an oxazoline compound such as oxazolidinone; a polyaminecompound; an oxazoline compound; a mono-, di- or poly-oxazolidinonecompound; a cyclic urea compound; and the like.

According to one embodiment, the surface cross-linking agent may be thesame as the internal cross-linking agent. For example, the surfacecross-linking agent may include a polyvalent epoxy compound of divalentor higher such as ethylene glycol diglycidyl ether. Alternatively, onlyethylene glycol diglycidyl ether may be used as the surfacecross-linking agent. The ethylene glycol diglycidyl ether has high watersolubility and has two epoxy groups, so that the surface reaction caneasily proceed even at a relatively low temperature.

The content of the surface cross-linking agent may be appropriatelyselected depending on the type of the added surface cross-linking agentor reaction conditions. For example, it may be used in an amount ofabout 0.001 to about 5 parts by weight, preferably about 0.01 to about 3parts by weight parts, and more preferably about 0.05 to about 2 partsby weight based on 100 parts by weight of the base resin. When thecontent of the surface cross-linking agent is too small, the surfacecross-linking reaction hardly occurs, and when it exceeds 5 parts byweight based on 100 parts by weight of the base resin, absorptionproperties such as water retention capacity may be deteriorated due toexcessive surface cross-linking reaction.

In addition, the method of mixing the surface cross-linking agent withthe base resin powder is not particularly limited. For example, a methodof adding the surface cross-linking agent and the base resin powder in areactor for mixing, a method of spraying the surface cross-linking agentonto the base resin powder, or a method of mixing the base resin powderand the surface cross-linking agent while continuously providing them toa continuously operating mixer may be used.

When adding the surface cross-linking agent, water may be mixed togetherand added in the form of a surface cross-linking solution. When water isadded thereto, there is an advantage that the surface cross-linkingagent may be evenly dispersed in the polymer. At this time, amounts ofwater to be added may be properly controlled for the purposes ofinducing a uniform dispersion of the surface cross-linking agent,preventing an agglomeration phenomenon of the polymer powder, andoptimizing a surface penetration depth of the surface cross-linkingagent. For example, water may preferably be added in an amount of about1 to about 10 parts by weight based on 100 parts by weight of the baseresin.

According to one embodiment of the present disclosure, propylene glycolor propylene carbonate is included as a solvent in addition to water.Moreover, alcohol-based solvents, such as methanol, are not included.

The propylene glycol or propylene carbonate does not participate in thesurface cross-linking reaction and functions as a solvent. Accordingly,the surface cross-linking solution is slowly absorbed into the baseresin, thereby enabling uniform application. Alcohol-based solvents suchas methanol, which are generally used as a solvent for a surfacecross-linking solution, have a disadvantage of causing a bad odor.However, according to one embodiment of the present disclosure, theabove effects can be achieved without a characteristic odor by includingpropylene glycol or propylene carbonate as a solvent of the surfacecross-linking solution and excluding an alcohol-based solvent.

In addition, it has been confirmed that when propylene glycol orpropylene carbonate is included as a solvent instead of thealcohol-based solvent, a rewet phenomenon can be improved while both thewater retention capacity and the absorbency under pressure aremaintained at high levels.

At this time, amounts of propylene glycol or propylene carbonate to beadded may be properly controlled for the purposes of inducing a uniformdispersion of the surface cross-linking agent, preventing anagglomeration phenomenon of the polymer powder, and optimizing a surfacepenetration depth of the surface cross-linking agent. For example,propylene glycol or propylene carbonate may preferably be added in anamount of about 0.1 parts by weight or more, about 0.2 parts by weightor more, or about 0.3 parts by weight or more, and about 5 parts byweight or less, about 4 parts by weight or less, or 3 parts by weight orless based on 100 parts by weight of the base resin.

The surface cross-linking reaction is performed by heating the baseresin to which the surface cross-linking solution containing the surfacecross-linking agent and the solvent is added at a temperature of about100 to about 150° C., preferably about 110 to about 140° C. for about 15to about 80 minutes, preferably about 20 to about 70 minutes. When thecross-linking reaction temperature is less than 100° C., the surfacecross-linking reaction may not sufficiently occur. When it exceeds 150°C., propylene glycol or propylene carbonate included as a solvent mayparticipate in the surface cross-linking reaction, and an additionalsurface cross-linking reaction by these compounds may proceed. When thesurface cross-linking reaction by propylene glycol or propylenecarbonate proceeds, the surface cross-linking density increases and thusabsorbency may be significantly reduced.

The heating means for the surface cross-linking reaction is notparticularly limited. It is possible to provide a thermal media theretoor provide a heat source directly thereto. At this time, usable thermalmedia may be a heated fluid such as steam, hot air, hot oil, and thelike, but the present invention is not limited thereto. Furthermore, thetemperature of the thermal media provided thereto may be properlyselected in consideration of the means of the thermal media, heatingspeed, and target temperature of heating. Meanwhile, an electric heateror a gas heater may be used as the heat source provided directly, butthe present disclosure is not limited thereto.

After the surface cross-linked layer is formed on the surface of thebase resin as described above, an inorganic material may be furthermixed therewith.

The inorganic material may be, for example, at least one selected fromthe group consisting of silica, clay, alumina, silica-alumina composite,and titania, and preferably silica.

The inorganic material may be used in an amount of 0.01 parts by weightor more, 0.05 parts by weight or more, or 0.1 parts by weight or more,and 5 parts by weight or less, 3 parts by weight or less, or 1 parts byweight or less based on 100 parts by weight of the super absorbentpolymer.

Meanwhile, in order to control the properties of the super absorbentpolymer to be finally commercialized, a step of classifying the superabsorbent polymer obtained after the surface cross-linking stepaccording to the particle diameter may be further performed. Preferably,a polymer having a particle diameter of about 150 to about 850 μm isclassified, and then only a super absorbent polymer having such aparticle diameter can be used as a final product.

The super absorbent polymer obtained by the above preparation method maysatisfy the following physical properties by achieving a balance betweenthe absorption rate (vortex time) and absorption properties.

-   -   1) a vortex time (absorption rate) at 24.0° C. is 35 seconds or        less;    -   2) one-minute tap water absorbency, which is defined as a weight        of water absorbed in the super absorbent polymer in 1 minute        when 1 g of the super absorbent polymer is immersed in 2 L of        tap water and swollen for 1 minute, is 115 g or more;

3) centrifuge retention capacity (CRC) measured according to the EDANAWSP 241.3 is 28 to 35 g/g, and

-   -   4) absorbency under pressure (AUP) at 0.7 psi measured according        to the EDANA WSP 242.3 is 16 to 22 g/g.    -   5) an effective capacity (EFFC) calculated by the following        Equation 1 is 22 g/g or more:

Effective capacity (EFFC)={CRC+0.7 psi AUP}/2   [Equation 1]

In Equation 1,

CRC is centrifuge retention capacity (CRC) of the super absorbentpolymer measured according to the EDANA WSP 241.3, and

0.7 psi AUP is absorbency under pressure (AUP) at 0.7 psi of the superabsorbent polymer measured according to the EDANA WSP 242.3.

More specifically, the super absorbent polymer prepared by the abovemethod may have a vortex time (absorption rate) by a vortex method of 35seconds or less, 34 seconds or less, or 33 seconds or less at 24.0° C.In addition, as the lower vortex time can be evaluated as the better,the lower limit is theoretically 0 seconds, but may be 10 seconds ormore, 15 seconds or more, 18 seconds or more, or 20 seconds or more. Themethod for measuring the vortex time of the super absorbent polymer willbe described in more detail in the following experimental examples.

In addition, the super absorbent polymer may have one-minute tap waterabsorbency of 115 g or more, 120 g or more, or 125 g or more, and 200 gor less, 190 g or less, or 180 g or less, wherein the one-minute tapwater absorbency is defined as a weight of water absorbed in the superabsorbent polymer in 1 minute when 1 g of the super absorbent polymer isimmersed in 2 L of tap water and swollen for 1 minute.

In addition, the super absorbent polymer may have centrifuge retentioncapacity (CRC) measured according to the EDANA WSP 241.3 of 29 g/g ormore, or 30 g/g or more, and 34 g/g or less, or 33 g/g or less.

In addition, the super absorbent polymer may have absorbency underpressure (AUP) at 0.7 psi measured according to the EDANA WSP 242.3 of18 g/g or more, or 19 g/g or more, and 22 g/g or less, or 20 g/g orless.

In addition, the super absorbent polymer may have an effective capacity(EFFC) calculated by the Equation 1 of 23 g/g or more, or 24 g/g ormore, and 28 g/g or less, 27 g/g or less, or 26 g/g or less.

The present invention will be described in more detail in the followingexamples. However, these examples are provided for illustrative purposesonly, and the content of the present invention is not limited by thefollowing examples.

Preparation Examples

The aqueous dispersion of hydrophobic particles and the encapsulatedfoaming agent used in the following Examples were prepared in thefollowing manner.

Preparation Example 1: Preparation of Aqueous Dispersion of CalciumStearate Ca-st

First, 50 g of water containing two or more surfactants (includingpolyoxyethylene alkyl ether-type nonionic surfactant and sulfate-typeanionic surfactant) was added to a high shear mixer, and heated to 165°C., followed by adding 50 g of calcium stearate powder. Then, it wasstirred for 30 minutes at 4000 rpm under normal pressure so that thecalcium stearate could be sufficiently pulverized to obtain an aqueousdispersion Ca-st in which 50 wt % of calcium stearate having an averageparticle diameter of 5 μm was dispersed. At this time, the pH of theaqueous dispersion was 9.5. In addition, after the Ca-st was prepared,its average particle diameter (D50) was measured/calculated using alaser diffraction particle size measuring device (Microtrac S3500) asthe particle diameter at 50% of the cumulative distribution of thenumber of particles.

Preparation Example 2: Preparation of Aqueous Dispersion of ZincStearate Zn-st

First, 70 g of water containing two or more surfactants (includingpolyoxyethylene alkyl ether-type nonionic surfactant and sulfate-typeanionic surfactant) was added to a high shear mixer, and heated to 140°C., followed by adding 30 g of zinc stearate powder. Then, it wasstirred for 30 minutes at 4000 rpm under normal pressure so that thezinc stearate could be sufficiently pulverized to obtain an aqueousdispersion Zn-st in which 30 wt % of zinc stearate having an averageparticle diameter of 1 μm was dispersed. At this time, the pH of theaqueous dispersion was 9.5, and the average particle diameter of Zn-stwas measured/calculated in the same manner as in Preparation Example 1.

Preparation Example 3: Preparation of Encapsulated Foaming Agent

As an encapsulated foaming agent used in Examples, F-36D manufactured byMatsumoto, which has a core of iso-butane and a shell of a copolymer ofacrylate and acrylonitrile, was prepared. At this time, the foamingstart temperature (T_(start)) of the F-36D is 70° C. to 80° C., and themaximum foaming temperature (T_(max)) is 110° C. to 120° C.

The diameter of each encapsulated foaming agent was measured as anaverage Feret diameter with an optical microscope. Then, an averagevalue of the diameters of the encapsulated foaming agents was obtainedand defined as the average diameter of the encapsulated foaming agent.

In addition, in order to confirm expansion characteristics of theencapsulated foaming agent, 0.2 g of the encapsulated foaming agentprepared above was applied on a glass Petri dish, and then left on a hotplate preheated to 150° C. for 10 minutes. The encapsulated foamingagent expanded slowly by heat, and this was observed with an opticalmicroscope to determine the maximum expansion ratio and maximumexpansion size of the encapsulated foaming agent in air.

A diameter of the top 10 wt % of the highly expanded particles afterapplying heat to the encapsulated foaming agent was defined as themaximum expansion size, and a ratio (D_(M)/D₀) of the average diameter(D_(M)) of the top 10 wt % of the highly expanded particles afterapplying heat to the average diameter (D₀) measured before applying heatto the encapsulated foaming agent was defined as the maximum expansionratio.

The average diameter of the prepared encapsulated foaming agent beforeexpansion was 13 μm, the maximum expansion ratio in air was about 9times, and the maximum expansion size was about 80 to 150 μm.

EXAMPLES Example 1

(Step 1) In a 3L glass container equipped with a stirrer and athermometer, 100 g of acrylic acid, 0.001 g of PEGDA 400 (polyethyleneglycol diacrylate 400) as an internal cross-linking agent, 0.27 g ofethylene glycol diglycidyl ether (EJ1030s), 0.008 g ofdiphenyl(2,4,6-trimethylbenzoyl)-phosphine oxide (I-819) as aphotoinitiator and 0.2 g of sodium persulfate as a thermal initiatorwere added and dissolved. Then, 188 g of 22% sodium hydroxide solutionwas added to prepare a monomer composition (degree of neutralization: 75mol %; solid content: 41 wt %).

(Step 2) 0.18 g of the aqueous dispersion of calcium stearate Ca-stprepared in Preparation Example 1 was added to the monomer compositionsuch that 0.09 g of calcium stearate was added based on 100 g of acrylicacid, and 0.06 g of the encapsulated foaming agent (F-36D) was addedthereto. Thereafter, the monomer composition was supplied at 500 to 2000mL/min on a conveyor belt in which a belt having a width of 10 cm and alength of 2 m was rotated at 50 cm/min. And, at the same time as thesupply of the monomer composition, ultraviolet rays having an intensityof 10 mW/cm² were irradiated to perform the polymerization reaction for60 seconds, thereby obtaining a sheet-type hydrogel polymer having amoisture content of 55 wt %.

(Step 3) Subsequently, the hydrogel polymer in the form of a sheet wascut to a size of about 5 cm×5 cm, and then pulverized in a meat chopperto obtain hydrogel particle crumbs having a size of 1 mm to 10 mm.Thereafter, the crumbs were dried in an oven capable of changing winddirection up and down. Thereafter, it was uniformly dried by flowing hotair at 185° C. or higher from the bottom to the top for 15 minutes, andthen flowing from the top to the bottom for 15 minutes, and the moisturecontent of the dried crumbs was set to 2% or less. After drying,pulverization was performed with a pulverizing machine, followed byclassification to prepare a base resin having a diameter of 150 to 850μm.

(Step 4) A surface cross-linking solution was prepared by mixing 4 g ofwater, 1.0 g of propylene glycol, 0.08 g of ethylene glycol diglycidylether, and 0.05 g of a polycarboxylic acid-based copolymer (a copolymerof methoxy polyethylene glycol monomethacrylate and methacrylic acid,Mw=40,000) based on 100 g of the base resin. The surface cross-linkingsolution was sprayed on 100 g of the obtained base resin powder formixing, put into a container consisting of a stirrer and a doublejacket, and the surface cross-linking reaction was performed at 140° C.for 35 minutes. Thereafter, the surface-treated powder was classifiedwith a ASTM standard mesh to obtain a super absorbent polymer powderhaving a particle diameter of 150 to 850 μm. Then, 0.1 g of fumed silica(AEROSIL® 200) was further mixed based on 100 g of the obtained polymerpowder.

Example 2

A super absorbent polymer was prepared in the same manner as in Example1, except that 0.12 g of the encapsulated foaming agent (F-36D) wasadded in Example 1.

Example 3

A super absorbent polymer was prepared in the same manner as in Example1, except that 0.3 g of the encapsulated foaming agent (F-36D) was addedin Example 1.

Example 4

A super absorbent polymer was prepared in the same manner as in Example1, except that 0.12 g of the encapsulated foaming agent (F-36D) wasadded in Example 1, and 0.3 g of the aqueous dispersion of zinc stearateZn-st prepared in Preparation Example 2 was added instead of 0.18 g ofCa-st as the aqueous dispersion of hydrophobic particles so that 0.09 gof zinc stearate was added based on 100 g of acrylic acid.

Example 5

A super absorbent polymer was prepared in the same manner as in Example2, except that 0.03 g (SDS 0.0084 g) of 28% sodium dodecyl sulfate (SDS)solution (ELOTANTE™ SL130, manufactured by LG Household & Health Care)as a foam stabilizer was further added to the monomer compositiontogether with the aqueous dispersion of hydrophobic particles and theencapsulated foaming agent in the step 2 of Example 2.

Example 6

A super absorbent polymer was prepared in the same manner as in Example3, except that 0.03 g (SDS 0.0084 g) of 28% sodium dodecyl sulfate (SDS)solution (ELOTANTE™ SL130, manufactured by LG Household & Health Care)as a foam stabilizer was further added to the monomer compositiontogether with the aqueous dispersion of hydrophobic particles and theencapsulated foaming agent in the step 2 of Example 3.

Example 7

A super absorbent polymer was prepared in the same manner as in Example2, except that 0.03 g of polyoxyethylene lauryl ether (LE-6,manufactured by Hannong Chemicals) as a foam stabilizer was furtheradded to the monomer composition together with the aqueous dispersion ofhydrophobic particles and the encapsulated foaming agent in the step 2of Example 2.

Example 8

A super absorbent polymer was prepared in the same manner as in Example3, except that 0.03 g of polyoxyethylene lauryl ether (LE-6,manufactured by Hannong Chemicals) as a foam stabilizer was furtheradded to the monomer composition together with the aqueous dispersion ofhydrophobic particles and the encapsulated foaming agent in the step 2of Example 3.

Comparative Example 1

A super absorbent polymer was prepared in the same manner as in Example1, except that the aqueous dispersion of hydrophobic particles was notused in Example 1.

Comparative Example 2

A super absorbent polymer was prepared in the same manner as in Example2, except that the aqueous dispersion of hydrophobic particles was notused in Example 2.

Comparative Example 3

A super absorbent polymer was prepared in the same manner as in Example3, except that the aqueous dispersion of hydrophobic particles was notused in Example 3.

Comparative Example 4

A super absorbent polymer was prepared in the same manner as in Example5, except that the aqueous dispersion of hydrophobic particles was notused in Example 5.

Comparative Example 5

A super absorbent polymer was prepared in the same manner as in Example7, except that the aqueous dispersion of hydrophobic particles was notused in Example 7.

Comparative Example 6

A super absorbent polymer was prepared in the same manner as in Example1, except that 0.09 g of calcium stearate (manufactured by Duksanscience) in the form of a powder having an average particle diameter of5 μm was used based on 100 g of acrylic acid instead of the Ca-st in theform of an aqueous dispersion of hydrophobic particles in Example 1.However, calcium stearate in the form of a powder could not be dispersedin the monomer composition and remained agglomerated and floated on theneutralization solution, so that a super absorbent polymer having auniform pore structure was not prepared.

Experimental Example 1: Measurement of Physical Properties of SuperAbsorbent Polymer

The physical properties of super absorbent polymers prepared in Examplesand Comparative Examples were evaluated in the following manner, and areshown in Table 1 below. Unless otherwise indicated, all procedures wereconducted in a constant temperature and humidity room (23±0.5° C.,relative humidity of 45±0.5%). In order to prevent measurement errors,an average value of three measurements was taken as measurement data. Inaddition, physiological saline or saline used in the evaluation of thefollowing physical properties means a 0.9 wt % sodium chloride (NaCl)aqueous solution.

(1) Centrifuge Retention Capacity (CRC)

The centrifuge retention capacity by absorption ratio under anon-loading condition of each polymer was measured according to theEDANA WSP 241.3.

Specifically, after inserting W₀ (g, about 0.2 g) of the super absorbentpolymer uniformly in a nonwoven fabric envelope and sealing the same, itwas soaked in saline (0.9 wt %) at room temperature. After 30 minutes,the envelope was centrifuged at 250 G for 3 minutes to drain, and theweight W₂ (g) of the envelope was measured. Further, after carrying outthe same operation without using the resin, the weight W₁ (g) of theenvelope was measured. Then, CRC (g/g) was calculated by using theobtained weight values according to the following Equation 2.

CRC (g/g)={[W₂(g)−W₁(g)]/W₀(g)}−2   [Equation 2]

(2) Absorbency Under Pressure (AUP)

The absorbency under pressure at 0.7 psi of each polymer was measuredaccording to the EDANA WSP 242.3.

Specifically, a 400 mesh stainless steel screen was installed in acylindrical bottom of a plastic having an inner diameter of 60 mm. W₀(g, 0.90 g) of the super absorbent polymer was uniformly scattered onthe screen at room temperature and a humidity of 50%. Thereafter, apiston which can uniformly provide a load of 0.7 psi was placed thereon.Herein, the outer diameter of the piston was slightly smaller than 60mm, there was no gap with the inner wall of the cylinder, and jig-jog ofthe cylinder was not interrupted. At this time, the weight W₃ (g) of thedevice was measured.

Subsequently, a glass filter having a diameter of 90 mm and a thicknessof 5 mm was placed in a petri dish having a diameter of 150 mm, andsaline (0.9 wt % sodium chloride) was poured in the dish. At this time,the saline was poured until the surface level of the saline became equalto the upper surface of the glass filter. One sheet of filter paper witha diameter of 90 mm was placed thereon. After the measuring device wasplaced on the filter paper, the liquid was absorbed for 1 hour under aload. After 1 hour, the measuring device was lifted, and the weight W₄(g) was measured.

Then, absorbency under pressure (g/g) was calculated by using theobtained weight values according to the following Equation 3.

AUP(g/g)=[W₄(g)−W₃(g)]/W₀(g)−3   [Equation 3]

(3) Vortex Time (Absorption Rate)

The vortex time (absorption rate) of the super absorbent polymers ofExamples and Comparative Examples was measured in the following manner.

-   -   {circle around (1)} First, 50 mL of 0.9% saline was added to a        100 mL beaker with a flat bottom using a 100 mL Mass Cylinder.    -   {circle around (2)} Next, after placing the beaker in the center        of a magnetic stirrer, a magnetic bar (8 mm in diameter, 30 mm        in length) was put in the beaker.    -   {circle around (3)} Thereafter, the stirrer was operated such        that the magnetic bar stirred at 600 rpm, and the lowermost part        of vortex generated by the stirring was made to reach the top of        the magnetic bar.    -   {circle around (4)} After confirming that the temperature of the        saline in the beaker reached 24.0° C., 2±0.01 g of a super        absorbent polymer sample was added and a stopwatch was operated        at the same time. Then, the time taken until the vortex        disappeared and a surface of liquid became completely horizontal        was measured in seconds, and this was taken as the vortex time.

(4) One-Minute Tap Water Absorbency

The one-minute tap water absorbency of the super absorbent polymers ofExamples and Comparative Examples was measured in the following manner.

-   -   {circle around (1)} First, 1 g of a super absorbent polymer was        put into a tea bag having a width of 15 cm and a length of 30        cm, and the tea bag was immersed in 2 L of tap water and then        swollen for 1 minute. {circle around (2)} Thereafter, the tea        bag containing the swollen super absorbent polymer was lifted        out of tap water. After 1 minute, the weight of the tea bag        without tap water was measured together with the weight of the        super absorbent polymer, and a value obtained by subtracting the        weight of the empty tea bag from the above weight was used as        the one-minute tap water absorbency.

At this time, the tap water used had an electrical conductivity of 170to 180 μS/cm when measured using Orion Star A222 (manufactured by ThermoScientific).

(5) Effective Capacity (EFFC)

The effective capacity (EFFC) was calculated by the Equation 1 with thecentrifuge retention capacity (CRC) and the absorbency under pressure(AUP) at 0.7 psi measured above.

TABLE 1 Aqueous Properties of super absorbent polymer dispersion ofone-minute Foam hydrophobic Foaming Vortex tap water stabilizerparticles agent CRC AUP EFFC time absorbency (content¹⁾) (content¹⁾)(content¹⁾) (g/g) (g/g) (g/g) (sec) (g) Example 1 — Aqueous F-36D 31.619.3 25.5 32 117 dispersion (0.06) Ca-st (0.18) Comparative — — F-36D32.1 19.8 26.0 51  85 Example 1 (0.06) Example 2 — Aqueous F-36D 31.419.0 25.2 28 132 dispersion (0.12) Ca-st (0.18) Example 4 Aqueous F-36D31.5 18.9 25.2 29 129 dispersion (0.12) Zn-st (0.3) Example 5 SDSAqueous F-36D 31.1 19.1 25.1 28 138 (0.0084) dispersion (0.12) Ca-st(0.18) Example 7 LE-6 Aqueous F-36D 31.2 18.8 25.0 27 140 (0.03)dispersion (0.12) Ca-st (0.18) Comparative — — F-36D 31.7 19.6 25.7 46 90 Example 2 (0.12) Comparative SDS — F-36D 31.3 19.3 25.3 32 110Example 4 (0.0084) (0.12) Comparative LE-6 — F-36D 31.1 19.2 25.2 32 113Example 5 (0.03) (0.12) Comparative — Ca-st F-36D 31.0 18.5 24.8 48  87Example 6 powder (0.12) (0.09) Example 3 — Aqueous F-36D 30.7 18.7 24.725 158 dispersion (0.3) Ca-st (0.18) Example 6 SDS Aqueous F-36D 30.418.1 24.3 23 168 (0.0084) dispersion (0.3) Ca-st (0.18) Example 8 LE-6Aqueous F-36D 30.3 18.2 24.3 22 171 (0.03) dispersion (0.3) Ca-st (0.18)Comparative — — F-36D 30.9 18.6 24.8 29 130 Example 3 (0.3) ¹⁾parts byweight based on 100 parts by weight of acrylic acid monomer

Referring to Table 1 above, it was confirmed that the super absorbentpolymers of Examples prepared by using the aqueous dispersion ofhydrophobic particles in the polymerization step exhibited a fastabsorption rate (vortex time) and improved one-minute tap waterabsorbency without deterioration in effective capacity, compared to thesuper absorbent polymers of Comparative Examples prepared without usingthe aqueous dispersion of hydrophobic particles or using only the foamstabilizer while using the same amount of the encapsulated foamingagent. In particular, it was found that the super absorbent polymer ofExample 2 showed significantly improved absorption rate (vortex time)and one-minute tap water absorbency together with improved effectivecapacity, compared to the super absorbent polymer of Comparative Example6 in which the hydrophobic particles are used in the form of a powderwhile using the same amount of the encapsulated foaming agent.

In addition, it could be confirmed that when the foam stabilizer wasused together with the encapsulated foaming agent and the aqueousdispersion of hydrophobic particles in the polymerization step, theabsorption rate (vortex time) could be further improved.

Accordingly, it was confirmed that when cross-linking polymerization ofa monomer was performed in the presence of an aqueous dispersion ofhydrophobic particles, it was possible to prepare a super absorbentpolymer with a significantly increased absorption rate (vortex time) byeffectively capturing the gas generated by the foaming agent.

1. A method for preparing a super absorbent polymer, comprising thesteps of: step 1: preparing a monomer composition containing an acrylicacid-based monomer having at least partially neutralized acidic groupsand an internal cross-linking agent; step 2: preparing a hydrogelpolymer by cross-linking polymerization of the monomer composition inthe presence of an aqueous dispersion of hydrophobic particles and anencapsulated foaming agent; step 3: forming a powder-type base resin bydrying and pulverizing the hydrogel polymer; and step 4: forming asurface cross-linked layer by further cross-linking the surface of thepowder-type base resin in the presence of a surface cross-linking agent,wherein the aqueous dispersion of hydrophobic particles is a colloidalsolution in which hydrophobic particles are dispersed by a surfactant,the hydrophobic particles contain a metal salt of a C7 to C24 fattyacid, and the encapsulated foaming agent has a structure having a corecomprising a hydrocarbon and a shell formed of a thermoplastic resinsurrounding the core.
 2. The method for preparing a super absorbentpolymer of claim 1, wherein the super absorbent polymer satisfies thefollowing 1) and 2): 1) a vortex time at 24.0° C. is 35 seconds or less;and 2) one-minute tap water absorbency, which is defined as a weight ofwater absorbed in the super absorbent polymer in 1 minute when 1 g ofthe super absorbent polymer is immersed in 2 L of tap water and swollenfor 1 minute, is 115 g or more.
 3. The method for preparing a superabsorbent polymer of claim 1, wherein the hydrophobic particles are atleast one metal salt of stearic acid selected from the group consistingof calcium stearate, magnesium stearate, sodium stearate, zinc stearateand potassium stearate.
 4. The method for preparing a super absorbentpolymer of claim 1, wherein the hydrophobic particles have an averageparticle diameter of 0.5 μm to 20 μm.
 5. The method for preparing asuper absorbent polymer of claim 1, wherein the hydrophobic particlesare used in an amount of 0.005 to 0.4 parts by weight based on 100 partsby weight of the acrylic acid-based monomer.
 6. The method for preparinga super absorbent polymer of claim 1, wherein the surfactant comprises anonionic surfactant and an anionic surfactant.
 7. The method forpreparing a super absorbent polymer of claim 1, wherein the encapsulatedfoaming agent has an average diameter before expansion of 5 to 30 μm,and a maximum expansion ratio in air of 5 to 15 times.
 8. The method forpreparing a super absorbent polymer of claim 1, wherein the encapsulatedfoaming agent has a maximum expansion size in air of 20 to 190 μm. 9.The method for preparing a super absorbent polymer of claim 1, whereinthe hydrocarbon is at least one selected from the group consisting ofn-propane, n-butane, iso-butane, cyclobutane, n-pentane, iso-pentane,cyclopentane, n-hexane, iso-hexane, cyclohexane, n-heptane, iso-heptane,cycloheptane, n-octane, iso-octane and cyclooctane.
 10. The method forpreparing a super absorbent polymer of claim 1, wherein thethermoplastic resin is a polymer formed from at least one monomerselected from the group consisting of a (meth)acrylate-based compound, a(meth) acrylonitrile-based compound, an aromatic vinyl compound, a vinylacetate compound and a halogenated vinyl compound.
 11. The method forpreparing a super absorbent polymer of claim 1, wherein the encapsulatedfoaming agent and the hydrophobic particles are used in a weight ratioof 1:0.1 to 1:3.5.
 12. The method for preparing a super absorbentpolymer of claim 1, wherein in step 2, the cross-linking polymerizationis performed in the presence of the aqueous dispersion of hydrophobicparticles, the encapsulated foaming agent, and further at least one foamstabilizer selected from the group consisting of an alkyl sulfate-basedcompound and a polyoxyethylene alkyl ether-based compound.
 13. Themethod for preparing a super absorbent polymer of claim 12, wherein theencapsulated foaming agent and the foam stabilizer are used in a weightratio of 1:0.01 to 1:0.5.
 14. The method for preparing a super absorbentpolymer of claim 1, wherein the surface cross-linking agent is the sameas the internal cross-linking agent.
 15. The method for preparing asuper absorbent polymer of claim 1, wherein the super absorbent polymerhas an effective capacity (EFFC) calculated by the following Equation 1of 22 g/g or more:Effective capacity (EFFC)={CRC+0.7 psi AUP}/2   [Equation 1] in Equation1, CRC is centrifuge retention capacity (CRC) of the super absorbentpolymer measured according to EDANA WSP 241.3, and 0.7 psi AUP isabsorbency under pressure (AUP) at 0.7 psi of the super absorbentpolymer measured according to EDANA WSP 242.3.
 16. The method forpreparing a super absorbent polymer of claim 1, wherein the acrylicacid-based monomer is included in an amount of about 20 to 60 wt % basedon a total weight of the monomer composition.
 17. The method forpreparing a super absorbent polymer of claim 1, wherein the hydrophobicparticles are included in an amount of 10 to 70 wt % based on a totalweight of the aqueous dispersion.
 18. The method for preparing a superabsorbent polymer of claim 1, wherein the aqueous dispersion ofhydrophobic particles has a pH of 7 or more.
 19. The method forpreparing a super absorbent polymer of claim 1, wherein the encapsulatedfoaming agent is included in an amount of 0.005 to 1 part by weightbased on 100 parts by weight of the acrylic acid-based monomer.
 20. Themethod for preparing a super absorbent polymer of claim 1, wherein thesuper absorbent polymer has a centrifuge retention capacity (CRC)measured according to EDANA WSP 241.3 of 28 to 35 g/g, and an absorbencyunder pressure (AUP) at 0.7 psi measured according to EDANA WSP 242.3 of16 to 22 g/g.